They didn’t have cars until they were forced off the land they had farmed for generations. Then they sold almost all of their possessions at rock bottom prices to buy worn out junk cars that might, if they were mechanically inclined, just about make it to California - where they would be exploited by the land owners.

In 1930, there was a car for every six people in the USA. Car ownership was not some absurdly rare thing, and many of the farms that failed were midwestern lands brought relatively recently into cultivation.

No argument about the exploitation by the land owners, but car ownership was genuinely common in the USA at the period.